David Cameron's cuts to green schemes will mean more than 400,000 badly insulated households miss out on help to reduce their bills, official figures show.

The government was accused by Labour of trying to "bury the truth" as ministers have repeatedly insisted that households would not lose out on energy-saving measures as a result of changes announced in December.

Cameron announced the cuts after having reportedly told colleagues he wanted them to get rid of the "green crap". The watering down of the Energy Company Obligation (ECO) – regulations forcing companies to pay for the installation of green measures – is expected to lead to a saving of £30 to £35 on household gas and electricity bills where companies choose to pass on the savings.

But official figures show it will also mean 440,000 fewer homes get help from the government to bring down their long-term energy costs through measures such as insulation and efficient boilers.

This appears to contradict the claims of Ed Davey, the Lib Dem energy secretary, who told parliament: "As a result of our changes, we believe that more ECO measures will help more households."

Greg Barker, a Tory energy minister, claimed that an additional 930,000 households would be helped as a result of the government's changes.

Labour's Caroline Flint, the shadow energy and climate change minister, said it was "a shameful attempt to bury the truth".

"For ministers to pretend that more people will receive help, when their own impact assessment shows exactly the opposite, is dishonest," she said.

"The government should be straight with people and admit that because of their deal with their energy companies, over 400,000 households are going to miss out on vital home improvements that would have cut their energy bills."

Energy companies are likely to benefit to the tune of millions of pounds from the extra gas and electricity sold to households that will no longer get help to reduce their usage. Some of the big six firms are still lobbying for further cuts to environmental levies on bills.

The Department of Energy and Climate Change (DECC) justified the claims of Davey and Barkerby arguing that the changes to the ECO scheme announced in December included a promise to extend it for another two years from 2015 to 2017. However, this argument is undermined by the official impact assessment, which states that the original scheme was designed on the assumption that it would continue at roughly the same level for around 10 years.

"In introducing the original phase of ECO beginning 2013, the government agreed, and made clear in the consultation process, that it expected ECO to continue at a broadly similar level of ambition until at least the end of 2022," this states.

Asked about the discrepancy, a DECC spokesman said: "In December, the government announced an additional £540m to make Britain's homes more energy-efficient, alongside changes to the ECO scheme, which protected the help available to vulnerable households and extended the scheme for an extra two years.

"Those changes will ensure that energy efficiency measures are more effectively targeted at homes which will benefit most, alongside a range of incentives for people to make permanent improvements which will keep their homes warmer and their bills lower."

They insisted the number of measures delivered to low-income homes and areas would be essentially unchanged.